replace multi_json dependency with regular stdlib JSON

JSON has been part of the ruby stdlib since 1.9.0 (see
https://github.com/ruby/ruby/commit/af1c4167), and the latest version of
google-api-client requires ruby 2.0 or better so we can assume it's available

lib/google/api_client/client_secrets.rb

@@ -12,8 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-
-require 'compat/multi_json'
+require 'json'
 
 
 module Google
@@ -71,7 +70,7 @@ module Google
             search_path = File.expand_path(File.join(search_path, '..'))
           end
         end
-        data = File.open(filename, 'r') { |file| MultiJson.load(file.read) }
+        data = File.open(filename, 'r') { |file| JSON.load(file.read) }
         return self.new(data)
       end
 
@@ -119,7 +118,7 @@ module Google
       # @return [String]
       #   JSON
       def to_json
-        return MultiJson.dump(to_hash)
+        return Json.dump(to_hash)
       end
 
       def to_hash